PHARM/PCS V5 · Procedure 20
Enzyme Kinetics III: Noncompetitive Inhibition
Get KI for an inhibitor that lowers Vmax without changing the enzyme's apparent affinity for substrate.
What it computes
A noncompetitive inhibitor binds at a site distinct from the active site, reducing Vmax without affecting substrate binding. On a Lineweaver-Burk plot, both slope and intercept increase by the same factor (1 + [I]/KI), and the two lines share the same x-intercept (−1/KM). KI is computed from the mean of the slope ratio and intercept ratio.
Inputs and outputs
You provide
- Uninhibited data: substrate concentration and velocity, one pair per line
- A second block of the same measurements with inhibitor present
- The inhibitor concentration [I]
PHARM/PCS reports
- Slope, Vmax, and KM for each line
- Slope ratio and intercept ratio, reported separately
- KI, from the mean of the two ratios
- The Lineweaver-Burk plot with the inhibited and uninhibited lines
When to use it
When an inhibitor lowers Vmax without changing the enzyme's apparent affinity for substrate, the diagnostic being lines that pivot around a shared x-intercept. Use it alongside Procedures 18 and 19 to distinguish inhibition types.
Reading the results
The tell is a common x-intercept (unchanged KM) with slope and intercept scaled up together. If the slope-ratio and intercept-ratio estimates of KI disagree markedly, the inhibition is likely mixed rather than cleanly noncompetitive, and the averaged KI is only a rough summary.
Diagnostic ratios
For pure noncompetitive inhibition the slope ratio and the intercept ratio should agree. The 1986 program printed only the final KI. This version shows both ratios, so a wide gap (2.51 against 1.87 in the book example) tells you the inhibition is probably mixed and the averaged KI is an approximation.
This page describes the procedure. Calculations run inside the licensed PHARM/PCS V5 application. PHARM/PCS is a trademark of MCS Scientific LLC.